In the manufacturing of a semiconductor device or a liquid crystal display device, the process of treating a member to be treated such as a wafer or a substrate having a microstructure formed on its surface is repeated. Achieving or maintaining high cleanliness in the treated member by removing contaminants stuck to the treated member is important for maintaining the quality of a final product or for improving manufacturing yield.
Recently, upgrading, high integration, and micronization of the treated member have progressed further in the manufacturing process of the semiconductor device and the liquid crystal display device. With this progress, the limitations of conventional wet (wet-type) cleaning treatment such as cleaning or drying by using ultrapure water or chemical solution has begun to be pointed out. To solve this problem, attention is now focused on a treatment device that performs cleaning or drying by using a supercritical fluid having such properties as low viscosity and low surface tension, especially supercritical carbon dioxide. The supercritical fluid, while it is similar in density to liquid, has low viscosity and high diffusibility and exhibits a behavior similar to that of gas. The supercritical fluid, which has properties of a high soaking force and easy contaminant diffusion, is suitable for cleaning the treated member having microstructure formed on its surface. No surface tension occurs in a supercritical state. Thus, in the drying process after cleaning, drying can be performed while preventing a collapse phenomenon caused by the capillary force of the fluid that remains on the treated member surface.
Substances that are employed for such supercritical fluids are carbon dioxide, nitrogen monoxide (N2O), sulfur dioxide (SO2), ethane (C2H6), propane (C3H8), chlorofluorocarbon, etc. In specially, the carbon dioxide has the advantages of being noncombustible and harmless and being easily handled because the critical temperature is about 31° C. and the critical pressure is about 7.4 MPa. Supercritical carbon dioxide (may be referred to as supercritical carbon dioxide gas) can be obtained by heating liquid carbon dioxide (may be referred to as liquefied carbon dioxide) that is compressed to a critical pressure or higher. Recently, studies have been conducted on the introduction of a cleaning or drying process using supercritical carbon dioxide in the semiconductor device manufacturing process. This necessitates a stable supplying of high purity carbon dioxide in which impurities and the number of particles (fine particles) have been reduced to the greatest possible degree.
JP 2006-326429 A (hereinafter, referred to as Patent Literature 1) discloses a system for supplying supercritical carbon dioxide while maintaining high cleanliness. In the system described in Patent Literature 1, carbon dioxide is purified through circulation treatment. The system described in Patent Literature 1 includes a circulation system that constantly circulates the purified carbon dioxide, and a supply system that supplies the supercritical carbon dioxide from the circulation system to a user point when necessary. The circulation system includes a condenser for liquefying gaseous carbon dioxide, and an evaporator/separator for gasifying the liquid carbon dioxide.